Energy expenditure plays a fundamental role in body weight regulation and in the prevention and treatment of obesity and other health problems such as cardiovascular disease. Results from the previous grant confirmed that the major factor altering energy expenditure is physical activity (PA) or exercise. There was a large individual variation in duration (CV-104%), among (CV=75.2%), and energy cost (CV=93.3%) of PA/exercise among the 152 subjects studied. The intra- and inter - individual variation, in type, duration, and intensity of PA/exercise is expected to be even grater in free-living conditions. Although epidemiological research (surveys, diaries, and simple mechanical/electronic monitors) suggests some potential impact of daily PA/acarus on weight regulation, previous experimental research has not successfully defined its nature, mainly due to the inability to quantify PA and its energy cost. Because of the complexities of human and because of diversified life style, it has been virtually impossible to accurately record free-livin PA/accurse on a daily basis. Quantifying PA refires numerous measurements and computations, and measurement in free-living requires the device to be small, non-invasive, reliable, easy to operate, and low in cost. Automation and computerization brought comfort to our life and reduced PA, which could contribute to the development of obesity. On the other hand, tremendous progress in microelectronics techniques makes it possible for the first time to quantitatively study daily PA and its relation with health problems such as obesity. We have designed and validated a portable microcomputer-based "smart insole" system to quantify daily PA under free-living conditions. We have also designed and validated a portable device for the estimation of energy expenditure in free-living. The existing who-room calorimeter and force platform system enable us to precisely calibrate these devices individually to ensure accurate measurements in free-living. Using these devices, we plan to study the duration, frequency, and intensity of various types of activities (sitting, lying down, standing, walking, running, step climbing, carrying weight etc) in 200 subjects with a wide range of characteristics. Intra- and inter- individual variations of thee activities and their energy cost will be investigated extensively over a long period. Variables relating to energy and macro nutrient balance, body composition, and fitness will also be examined. this enables us to study the relations between Pa/exercise and changes in body weight, composition, and fitness, and the potential determinants of PA and its energy cost. We believe these new portable devices, combined with the existing room calorimeter and large force platform, renders us a unique opportunity to quantitatively and reliably study daily PA and its impact on body weight regulation under free-living conditions.